The Ni-based superalloy tube C-HRA-1is manufactured under industrial process for 700 ℃ advanced ultra-supercritical unit boiler.The microstructural evolution of C-HRA-1tube during long-term thermal exposure at 750℃and 800℃ was investigated by optical microscope

scanning electron microscope(SEM)

transmission electron microscopy(TEM)and thermal calculation.The results show that

the coldrolled tube suffered fully recrystallization after solution treatment

and no obvious precipitation was observed.After age hardening treatment at 800℃/12h/AC

γ′phase with fine spherical was dispersively distributed and small amount of MC carbide particles precipitated in the matrix.In addition

M23C6 precipitated along the grain boundaries and the twinning boundaries.During long-term thermal exposure at 750 ℃and 800 ℃

theγ′phase size increased and its spherical morphology turned to cubical with increasing exposure time and exposure temperature.However

noηphase and other new precipitate was found during long term thermal exposure.It demonstrated that the microstucture stability of the C-HRA-1tube is excellent.